Semiconductor devices are commonly found in modern electronic products. Semiconductor devices vary in the number and density of electrical components. Discrete semiconductor devices generally contain one type of electrical component, e.g., light emitting diode (LED), small signal transistor, resistor, capacitor, inductor, and power metal oxide semiconductor field effect transistor (MOSFET). Integrated semiconductor devices typically contain hundreds to millions of electrical components. Examples of integrated semiconductor devices include microcontrollers, microprocessors, charged-coupled devices (CCDs), solar cells, and digital micro-mirror devices (DMDs).
Semiconductor devices perform a wide range of functions such as high-speed calculations, transmitting and receiving electromagnetic signals, controlling electronic devices, transforming sunlight to electricity, and creating visual projections for television displays. Semiconductor devices are found in the fields of entertainment, communications, power conversion, networks, computers, and consumer products. Semiconductor devices are also found in military applications, aviation, automotive, industrial controllers, and office equipment.
Semiconductor devices exploit the electrical properties of semiconductor materials. The atomic structure of semiconductor material allows its electrical conductivity to be manipulated by the application of an electric field or base current or through the process of doping. Doping introduces impurities into the semiconductor material to manipulate and control the conductivity of the semiconductor device.
A semiconductor device contains active and passive electrical structures. Active structures, including bipolar and field effect transistors, control the flow of electrical current. By varying levels of doping and application of an electric field or base current, the transistor either promotes or restricts the flow of electrical current. Passive structures, including resistors, capacitors, and inductors, create a relationship between voltage and current necessary to perform a variety of electrical functions. The passive and active structures are electrically connected to form circuits, which enable the semiconductor device to perform high-speed calculations and other useful functions.
Semiconductor devices are generally manufactured using two complex manufacturing processes, i.e., front-end manufacturing, and back-end manufacturing, each involving potentially hundreds of steps. Front-end manufacturing involves the formation of a plurality of die on the surface of a semiconductor wafer. Each die is typically identical and contains circuits formed by electrically connecting active and passive components. Back-end manufacturing involves singulating individual die from the finished wafer and packaging the die to provide structural support and environmental isolation.
One goal of semiconductor manufacturing is to produce smaller semiconductor devices. Smaller devices typically consume less power, have higher performance, and can be produced more efficiently. In addition, smaller semiconductor devices have a smaller footprint, which is desirable for smaller end products. A smaller die size may be achieved by improvements in the front-end process resulting in die with smaller, higher density active and passive components. Back-end processes may result in semiconductor device packages with a smaller footprint by improvements in electrical interconnection and packaging materials.
Another goal of semiconductor manufacturing is to produce higher performance semiconductor devices. Increases in device performance can be accomplished by forming active components that are capable of operating at higher speeds. In high frequency applications, such as radio frequency (RF) wireless communications, integrated passive devices (IPDs) are often contained within the semiconductor device. Examples of IPDs include resistors, capacitors, and inductors. A typical RF system requires multiple IPDs in one or more semiconductor packages to perform the necessary electrical functions.
An RF balun (balanced and unbalanced) is an important component in wireless communication systems. The RF balun is used to convert differential signals, e.g., from a power amplifier or transceiver, to single-end signal, with proper impedance transformation. The balun suppresses electrical noise, performs impedance transformation and matching, and minimizes common-mode (noise random noise or other electrical interference) through electromagnetic coupling.
A conventional RF balun 10 is shown in FIG. 1 with conductive trace or coil 12 inter-wound or interleaved with conductive trace or coil 14 to increase mutual coupling between the inductors. Conductive trace 12 has first and second end terminals coupled to balanced ports 16 and 18. Capacitor 20 is coupled between ports 16 and 18. The inductor 12 and capacitor 20 constitute a first LC (inductor and capacitor) resonator. Conductive trace 14 has first and second end terminals coupled to unbalanced port 24 and port 26 (ground terminal). Capacitor 28 is coupled between ports 24 and 26. The center-tap 30 and conductive trace 32 supplies a DC bias to balanced ports 16 and 18. The inductor 14 and capacitor 28 constitute a second LC resonator.
A figure of merit for the RF balun performance is common-mode rejection ratio (CMRR). An insufficient CMRR results in power supply modulation and self-mixing in receiver circuits, such as the low noise amplifier. The harmonic response of an electrical device, such as a power amplifier, often exist in common-mode form. To reduce the unwanted harmonic response, a high CMRR is desirable in the RF balun.
A high CMRR can be difficult to achieve with the implementation shown in FIG. 1, particularly at higher frequencies, due in part to capacitive coupling between the LC resonator 12 and 20 and the LC resonator 14 and 28. The currents in LC resonators are coupled by mutual inductance. An input signal to the unbalanced LC resonator induces current in the balanced LC resonator, and vice-versa. Ideally, a common-mode signal applied to the balanced ports 16 and 18 causes equal and opposite currents to flow in inductor 14 and no signal is transferred to the unbalanced port 24. To compensate, the size of the RF balun is made relatively large for strong magnetic coupling. The coupling coefficient between the LC resonators is typically made as large as practical, e.g., larger than 0.6, to achieve the requisite magnetic coupling. In addition, the capacitive coupling between the LC resonators is made large for greater bandwidth.
However, parasitic capacitive coupling between LC resonators allows leakage of the common-mode signal to the unbalanced port, particularly at higher frequencies. While the larger balun with interleaved conductive traces has certain advantages, i.e., robustness to manufacturing variation as well as improved bandwidth, pass-band response, matching, loaded Q, resistive losses, and insertion loss, it also consumes die area which adds cost to the manufacturing process, reduces balance, and increases capacitive coupling which decreases CMRR.